1. Field of the Invention
The present invention relates to a method of manufacturing a water-soluble expendable salt core for casting.
2. Description of the Related Art
As is well known, casting such as aluminum die casting is a technique of casting a structure having a desired shape by injecting a melt of an aluminum alloy into a metal mold at high speed and high pressure. In casting like this, a core is used to mold a cast product having a hollow structure, e.g., a water jacket for water cooling such as a cylinder block of an internal combustion engine. A core used in a case like this is apt to receive a large impact because a metal melt injected at high speed from a gate impacts against the core. In addition, the casting pressure is high until the completion of solidification. Therefore, the core is required to have strength that can withstand a high pressure and high temperature.
Also, as is well known, the core is removed from a cast product after casting. However, if a general sand expendable core solidified by a phenolic resin is used for a cast product having a complicated internal structure, it is not easy to remove the expendable core. On the other hand, water-soluble expendable salt cores removable by dissolution in high-temperature water or the like are disclosed in Japanese Patent Publication No. 48-039696, Japanese Patent Laid-Open No. 50-136225, and Japanese Patent Publication No. 52-010803. An expendable salt core is manufactured by melting and molding a salt mixture of, e.g., sodium carbonate (Na2CO3), potassium chloride (KCl), and sodium chloride (NaCl), thereby obtaining a high pressure resistance, and improving the workability and stability of casting.
As described above, an expendable salt core manufactured by melting and molding a salt mixture and having a high strength has been developed. However, expendable salt cores have large variations in strength, and hence have not completely been put into practical use.